Serial Attached SCSI (SAS) is a protocol used to move data to and from computer storage devices such as hard drives and tape drives. A SAS system includes initiators, targets, and a service delivery subsystem which may include expanders.
The initiator contains initiator ports and originates commands and task-management functions to be processed by a target device, and receives responses for the same requests. A target device contains logical units and target ports that process device-service and task-management requests. Each logical unit is assigned a logical unit number and a logical unit name within the target device. The target device may send responses through a target port and across a service delivery subsystem to the initiator device. A target device could be a hard disk, tape drive, or a disk array system.
A service delivery subsystem is that part of an input/output (I/O) system that transmits information between the initiator port and the target port. The service delivery subsystem includes board traces, cables, and may also include expanders. An expander facilitates communication between multiple SAS devices (e.g., the connection of multiple SAS end devices to a single initiator port).
Each device may contain one or more logical units that share the same ports and service delivery subsystem. The logical units may be the same type (e.g., a disk array may present many disk drives) or may be different types (e.g., a tape drive and a tape library may be presented by the same device). The same storage device (e.g., a tape library) may be presented through two different target devices as logical units, but the library is required to have different names for each connection so that they can be distinguished. Presenting the same storage device through two different paths using two different names is called multi-pathing. This multi-path connection may require a special software application installed between the application and the host bus adapters (HBAs) to convert the two paths into a single logical path. This application may manage which path is used for communication with the library.
A tape library may include a media changer device and one or more data transfer devices. A media changer using a SAS interface may have its SAS interface hosted by a data transfer device (e.g., a tape drive, optical disk drive, etc.). One possible connection method may be for the data transfer device to present two logical units to the initiator.
The logical view from the initiator may be a data transfer device with a media changer behind it. If the drive has to be removed or the drive fails and the communication link is lost to the drive, then the communication link to the media changer is also lost. If there are several drives in the library the rest of the drives may be functioning correctly but because commands cannot be sent to the media changer and no media can be loaded or unloaded, then the drives are unable to be used.